Studies On The Extraction And Separation Of Americium And Europium With Several N-donor Ligands

The sustainable development of nuclear energy requires not only the full utilization of uranium fuel resources,but also to avoid the impact on the survival environment of human beings.The traditional“once-through”strategy is not sustainable.Partition and Transmutation strategy can transform the high toxicity,long-lived radionuclides to low level radioactive waste in spent fuel,and enable large amounts of ²³⁸U to breed and generate electricity at the same time.It is the promising appoach to the advanced nuclear fuel cycle at present.However,in order to improve the neutron economy and increase the transmutation efficiency,the neutron poison must be eliminated at first.And it is necessary to separate the majority of minor actinides from the high content lanthanides.As the representative,the separation of americium and europium is one of the most important and challenging problem due to their similar radii and chemical properties.Based on the CHON principle and the current situation of the solvent extraction and Ln/An separation in high level liquid waste,we carried out the investigation on the separation of Am???and Eu???with several classical N-donor ligands.We studied the influence of molecular structure on the extraction and separation properity by the conditions of extraction,the mechanism of coordination in combination with the theoretical calculation.The main contents in this article are as follows:First,two kinds of di-picolinamide ligands bpb and bpp,containing both soft N and hard O atoms,are designed and synthesized.The experiments on Am???and Eu???extraction with bpb and bpp are carried out.The distribution ratios as well as the separation factors are compared in different acidity and ligand concentration.According to the infrared spectra,UV-vis spectroscopic titration and high resolution mass spectrometry of the complexes,we found that the separation property of the rigid bpp molecule is better than that of the flexible structure at low acidity.When pH is 5.0,the SF?Am/Eu?is up to 11.Though tetradentate N-donor bpp form 1:1 type complex with trivalent metal ions,the system exhibits poor acid resistance.However,the structure of flexible bpb is more difficult to protonate than that of rigid bpp,yet its extraction ability is a little weaker.The main species of the complexes is a 1:2 dinuclear compound,and the two metal ions coordinate with the heterocylic N and carbonyl O at both sides separately.The conclusion can intuitively compare the effect of rigidity and flexibility on the extraction and separation process,and can provide some significance for the subsequent experiment.Then,five bis-triazin-pyridine?BTP?derivatives with short alkyl chains are synthesized and characterized.The effect of protonation and steric hindrance are studied as the shortened side chains and the benzene rings change the rigidity and flexibility of the ligands.The C2-BTP with flexible branching chain is selected as the ligand with the best separation property,and the SF?Am/Eu?is up to 25 when the acidity is 1.25 M.Analysis of slope method,infrared spectrum,UV-vis spectroscopic titration and mass spectrometry of the complex reveal that the ligand has the coordinated N atoms on heterocyclic ring,and predominantly forms 2:1 complex with Am???and 1:1with Eu???.Besides,the novel C2-BTP/ionic liquid separation system is explored,and is proved to be a cation-exchange mechanism.The extraction reaction is a slow-motion process.However,the experimental and molecular dynamics method reveals that the kinetics and extractability,D?Am?and SF?Am/Eu?,can be remarkably improved with the increase of temperature.In the latest work,the tetradentate ligand EtTol-NTA is designed and synthesized.This ligand has a three-branch structure,with one tertiary amine N in the center.Compare to the dissolving defect of its cyclized derivative,this ligand has good extraction performances for Am???and Eu???in octanol,dichloroethane and nitrobenzene.Its D?Am?reaches about 100 and SF?Am/Eu?is up to 23.2 when the acidity is 0.05 M.Slope analysis,FTIR,UV spectrum and MS prove that the ligand predominantly form 2:1 complex with metal ions.In order to maintain electrical neutrality,only one nitrate is in the inner coordination sphere while the other two in the outer.The species of the complex is deduced to be M?NO₃??L?₂·?NO₃?₂.The calculated Gibbs free energy at 298 K is–10.77 kJ/mol and–14.38 kJ/mol for Am???and Eu???,respectively.This indicates that the coordination processes are exothermic.The once stripping efficiencies are all more than 80%at high acidity.All the results mean EtTol-NTA has a certain potential for industrial application,and it can be applied in the designing and optimization of room-temperature separation process in the future.In addition,the chemical regularity of the above-mentioned ligands and their corresponding complexes are investigated by the quantum chemistry and the density functional theory?DFT?.Combined with the experimental data,the relationship of the molecular structures with the separation property and the acid resistance are compared and analyzed from the aspects of geometry structure,electronic structure and energy.The conclusion is that the rigidity and flexibility of side chains in the ligands notably affect the micro coordination environment?the cavity size?with metal ions,and is one of the main factors on the Am???/Eu???separation peformance.It provides a feasible hint and direction for designing and modifying new efficient An/Ln separation extractants and can improve the novel sophisticated extraction systems in the future.